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How does Arctic sea ice loss compare to Antarctic sea ice gain?

What the science says...

Select a level... Basic Intermediate

Arctic sea ice loss is three times greater than Antarctic sea ice gain, and the amount of solar energy absorbed by the Earth is increasing as a result.

Climate Myth...

Arctic sea ice loss is matched by Antarctic sea ice gain
In fact, the global sea-ice record shows virtually no change throughout the past 30 years, because the quite rapid loss of Arctic sea ice since the satellites were watching has been matched by a near-equally rapid gain of Antarctic sea ice. Indeed, when the summer extent of Arctic sea ice reached its lowest point in the 30-year record in mid-September 2007, just three weeks later the Antarctic sea extent reached a 30-year record high. The record low was widely reported; the corresponding record high was almost entirely unreported. (Chris Monckton)

First of all, it is very simple to look at the data and realize that the Arctic is losing more sea ice than the very slight increase in Antarctic sea ice (Figures 1 and 2).

global sea ice 

Figure 1: Global sea ice extent since 1979. (Image source: Tamino. Data is from US National Snow and Ice Data Center.)

global

Figure 2: National Snow and Ice Data Center (NSIDC) Antarctic, Arctic, and global (sum of the two) sea ice extents with linear trends.  The data is smoothed with a 12-month running average.

Sea ice area data shows the same thing as extent data.

However, some have argued that Antarctic sea ice gain is more important than Arctic sea ice loss because they claim it has more effect on the amount of solar radiation absorbed by the Earth.  Is there any truth to this claim?

The Poles Receive More Solar Energy in Summer

First of all, for most of the year higher latitudes receive less solar energy than lower latitudes. But deep in the heart of summer this is no longer true. In fact at the point of midsummer, the poles receive more solar energy than any other place on earth. Here’s the solar insolation as a function of latitude, for various times of year from northern midwinter (southern midsummer) to northern midsummer (southern midwinter) (note: this does not include the correction for varying earth-sun distance, it’s merely to illustrate the latitude dependence at a given time of year):

insolation

During the summer months, when ice albedo feedback really counts, higher latitudes really do get more solar energy than lower latitudes. When the insolation depends strongly on latitude, with higher latitudes getting much less solar energy, the total insolation is so small that the impact of albedo change is small.

Northern Hemisphere Summer is Weaker but Longer than Southern

Another factor is that earth is farthest from the sun during northern hemisphere summer (when albedo change counts most) but closest to the sun in southern hemisphere summer. That has led some to believe that the impact of albedo change will be enhanced in the southern hemisphere. This is an understandable mistake; in southern hemisphere summer earth really does receive more solar energy per unit of time because we really are closer to the sun. But the rate of travel of the earth along its orbit is inversely proportional to the square of our distance from the sun — exactly the same proportion by which solar energy changes — so the northern hemisphere summer may be weaker than the southern, but it also lasts longer. For total insolation, the two factors cancel each other so at any given latitude, the annual-total solar energy input is unaffected (a fact which was emphasized by Peter Huybers in relation to ice age cycles). So no, proximity to the sun during southern hemisphere summer doesn’t enhance the ice albedo effect.

Let's Do the Math

If we really want to know the relative impact of ice albedo change between hemispheres, the thing to do is: the math.

I took daily values of sea ice area from NSIDC. I modelled the Arctic ice pack as a circle around the pole of the same area, and the Antarctic ice pack as a ring around the Antarctic continent, with the edge of the Antarctic continent as being at latitude 70°S. This is only a crude representation of the geometery of the sea ice packs, but it’s at least correct to first order.

Then I computed, for each day, the sun’s declination (its altitude above or below the celestial equator). I also computed the earth-sun distance so I could apply that correction to the solar insolation. This enabled me to compute the total amount of solar radiation hitting parts of the earth which are covered by sea ice, which I call “sea ice insolation“. I did so separately for the northern and southern hemispheres.

For both hemispheres, sea ice insolation shows a strong annual cycle. So I computed the average total sea ice insolation for each year, for each hemisphere, corrected for earth-sun distance and angle of elevation of the sun. This gives the annual average sea ice insolation in TW (teraWatts). We can then look at the trends in sea ice insolation, to see which pole has lost or gained more in terms of solar energy impacting sea-ice-covered regions.

It’s quite a complicated calculation, so it’s possible I’ve made an error. But the amounts are certainly in the right ballpark, and I’ve done orbital calculations for decades, so I suspect I got it right. The data cover the time span from 1979 through 2011.

And here’s the result: annual average sea ice insolation, together with linear trend lines, for both hemispheres:

ice insolation

You can see the trends. Clearly. The trend has shown an increase in Antarctic sea ice insolation of about 53 TW, and a decrease in Arctic sea ice insolation of about 329 TW. That’s over 6 times as great.

If we spread 53 TW over the entire earth we get a global average of 0.10 W/m^2. So even if the difference between ice and ocean albedo were equal to 1 in the southern hemisphere (i.e., Antarctic sea ice were perfectly reflective while ocean was completely absorbing) the net global climate forcing would amount to -0.10 W/m^2. But sea ice isn’t perfectly reflecting, not even in the southern hemisphere where the sea ice is often snow-covered, and ocean is not perfectly absorbing. If the top-of-atmosphere (TOA) albedo difference between sea-ice-covered and open ocean areas is 0.2, then the global climate forcing from Antarctic sea ice changes would be about -0.02 W/m^2.

If we spread 329 TW over the entire earth we get a global average of 0.65 W/m^2. If the TOA albedo difference between sea ice and open ocean is 0.2, then the global climate forcing from Arctic sea ice changes would be about +0.13 W/m^2.

And it turns out that 0.2 is not a bad figure for the TOA albedo difference, according to Hudson (2011):

TOA insolation

In fact Hudson states that

"Results show that the globally and annually averaged radiative forcing caused by the observed loss of sea ice in the Arctic between 1979 and 2007 is approximately 0.1 W m-2"

so my crude calculation is certainly in the right ballpark. As I said, this is a pretty complex calculation so I may have made an error, but I certainly ended up in the right neighborhood according to Hudson.  The combined increased radiative forcing caused by the increase in Antarctic sea ice and decrease in Arctic sea ice from 1979 to 2011 is approximately +0.1 W/m^2.

Last updated on 5 October 2012 by dana1981. View Archives

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Comments

Comments 1 to 9:

  1. Hope I've got the right thread here. I recently had an online discussion with a 'skeptic' in which he claimed that 'skeptics' only talk about sea ice extent because the measurements for volume are unreliable. I duly did my research and, as I expected, his criticisms were invalid. Measurements are taken by satellite, submarine, and by scientists on the ground,then extrapolated for the whole area of coverage. From what I read the results are reliable, in fact slightly overestimating the thickness of the ice. However, two days ago I read a newspaper article in which the chief scientist at the met office was quoted as saying that the thickness of arctic sea ice is not known with any confidence.
    http://www.guardian.co.uk/environment/2012/mar/14/met-office-arctic-sea-ice-loss-winter
    This has left me slightly baffled and perplexed. I just wondered if anyone would like to try and shine a light on this for me. Thanks.
    Response:

    [DB] This was discussed over at Neven's (starting here).  See also Arctic Sea Ice Hockey Stick: Melt Unprecedented in Last 1,450 years, where you will find this graphic (extent being an approximation of relative volumes over time):

    Click to enlarge

  2. Thanks for the link. From the discussion at Nevens its clear that Ms Slingos comments seem to be odds at what other scientists are saying. But I should imagine that her comments will be used by 'skeptics' to justify their exclusion of ice volume measurements and, in this case, they can point to a credible source to back up their argument.
  3. "Arctic sea ice loss is three times greater than Antarctic sea ice gain."
    ===
    That should be "three times as much as". It's different for the same reason that 50% more is different from 50% as much.
  4. Normal 0 false false false false EN-US X-NONE X-NONE

    The data presented above is outdated by a few years. The combined anomoly of NH and SH sea ice is trending upward since 2007. As electric rates have risen 40% over the last 7 years, justification has thinned more than the ice has.

     

    However, the Arctic ice volume 2014 peak is lower than the 2007 peak, but by a very small amount, far less than 1000 cubic km (PIOMAS). I'm not seeing the Arctic sea ice decrease on the order of 'three times as much as' in recent data. Things are different than in July 2012. This year has seen the remaining Arctic ice exceed seven other recent years just since April 22nd (Charctic), due to slow spring 2014 melt and the remaining Arctic ice is only 5.4% less than this time in 1994. As a comparison, 1994 was a very cold winter as I moved from S. Fla to Indiana at the start of that deadly winter. The temp at my apartment hit -44F without the wind chill on MLK day. I considered the possibility that the cold air over the great plains that day contracted the U.S. such as to have caused the Oakland earthquake that same day. As I recently have read about ocean volume thermal expansion due to surface temp increase, it might not have been so far fetched a thought.

    Response:

    [PS] This moderator is struggling to decide whether you actually cant understand that cherrypicking and short trends are not science or whether you are trolling. You have been called on this before.

  5. Jetfuel @4, let me see...

    1)  Use of short time span for comparison?  Only eight years of a 30 plus year record used.  Check!

    2)  Use of previous record breaking year as start point for comparison?  2007 record year used.  Check!

    Well, your certainly playing from the denier play sheet for bad science.

    Trying to turn an April PIOMAS which is less than the trend value into evidence that the trend is reducing certainly shows gumption, but surely you must know that such unethical distortions of the facts will get called on this site:

    Response:

    [PS] Jetfuel, please ackowledge that you understand Tom's point. If you dont understand, then more explanation is likely to be offered. If we get a "look, squirrel" instead, then your posts will be deleted.

  6. Response @4&5.

    The cherry-picked number,Δ(Arctic Sea Ice Volume maximum 2007 & 2014), described as "a very small amount, far less than 1000 cubic km (PIOMAS)" is 750 km3, which over a seven year period and in the units used in the graph @5 represents a trend of -1.07 (1000 km3/Decade). It isn't a very ripe cherry.

    The comparison presented @4, that of Arctic Sea Ice loss being allegedly not "on the order of 'three times as much as' in recent data," it is a comparison with Antarctic Sea Ice gain. Antarctic Sea Ice Volumes are not as well understood as their northern equivalents, but Holland et al (2014) suggest an Antarctic Sea Ice Volume trend of +0.3 (1000 km3/Decade) for 1992-2010. Ironically, that is about a third the size of the cherry-picked measure of ice loss in the Arctic.

  7. I do wonder if the attempt by jetfuel@4 to challenge the assertion made in this post (that Antarctic Sea Ice is growing at a much slower rate that Arctic Sea Ice is shrinking) should be batted away innto the long grass simply because of the incompetence of jetfuel to state his case.


    It is true that over the satellite record (1979 - to date), the decline in Arctic Sea Ice Area & Extent is roughly three times larger than the increase in Antarctic Se Ice Area & Extent. It is also true that both the Arctic decline and the Antarctic increase - both these trends have shown signs of acceleration over recent years, yet generally the three-to-one ratio remains. And it is true that the acceleration in trends is accompanied by greater variability but this has not resulted in net global Sea Ice area & extent anomalies reaching any unprecedented values* when examined as daily, monthly or quarterly averages, *unprecedented taken as values over the last decade, there being variability evident with such periodicity.
    However, recent Net Global Sea Ice area & extent do start to show unprecedented values* when Annual Averages are examined. This results mainly from the Antarctic anomaly showing a rising trend over the last two years. While this remains presently a short-term phenomenon, the mechanisms behind the growth of Antarctic Sea Ice area & extent are known to be the product of Antarctic regions with increasing sea ice cover and other Antarctic regions with decreasing sea ice cover. The net Antarctic anomaly is the result of two far larger values that presently come close to cancelling each other out. Such a cancellation cannot be relied on. One of the mechanisms (for increase or decrease) could easily come to dominate the anomaly in future decades, as may have been the case prior to the satellite era. Indeed, Fan et al (2014) argue quite convincingly that the start of the satellite data (1979) likely coincides with a shift from significant Antarctic summer (DJF) ice loss over the period 1950-78.
    Thus, while the comments by jetfuel are based on cherry-picking nonsense, and while the Net Global Sea Ice area & extent has been in decline over the satellite era (1979 - to date), predicting a continuation of that decline is potentially foolhardy as future trends, in particular Antarctic Sea Ice cover, remain uncertain.

  8. Is the scale on the Antarctic Sea Ice Extent graph in Figure 2 correct? I thought that Antarctic sea ice was up around the 20 million sq km mark. Thanks.

  9. Rett,

    The data in figure 2 is a 12 month average, not the maximun.  The Antarctic sea ice maximun (this year) was just over20 million km2.  Sea ice was only that high for about one week.  Sea ice has dropped to about 18 million km2.  The yearly average is lower than the maximum is.  The graph is also from last year so it does not include the most recent data.

    The yearly average is more informative than the maximum because it tells us about what is happeing the entire year.  A graph of all the data can be found here (the graph is area not extent so the maximun this year is 17 million km2.  The graph of the Antarctic is about half way down the page.)

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